The present invention relates generally to an improved high sensitivity optical inspection system and method for detecting flaws on a diffractive surface with pattern features, and more particularly to a system and method which differentiates between light scattered by a pattern on the surface and light scattered by a flaw.
Detection of flaws such as particles, holes, bumps, pits or fingerprints on a surface having diffractive features, such as on a photolithographic mask which is conventionally used in modern semi-conductor photolithography, or any other defect on a patterned surface hereinafter generically referred to as a xe2x80x9cplatexe2x80x9d, is critical to maintaining a high level of quality control.
A system which accomplishes this function is disclosed in U.S. Pat. No. 5,625,193 which is assigned to the same assignee as the instant application and is incorporated herein by reference in its entirety. The system disclosed in U.S. Pat. No. 5,625,193 includes a laser which provides a beam of ultraviolet laser light that is scanned across the entire surface of the plate. The angular intensity distribution sensed by an array of detectors in response to the illumination at each point on the plate surface is used to determine the location and size of flaws on the plate surface.
A need exists for a high sensitivity optical inspection system which differentiates between light scattered by a pattern on the surface of the plate, light scattered by a flaw on the surface the plate and system noise, which overcomes limitations and deficiencies of the prior art.
It is an object of the present invention to provide an improved high sensitivity optical inspection system for detecting and distinguishing between light scattered from flaws and light scattered from surface patterns defined on a diffractive surface.
Accordingly, the present invention sets forth an improved high sensitivity optical inspection system for detecting flaws on a diffractive surface containing surface patterns. The system includes at least one optical source that provides a first beam. The first beam illuminates a first region of the diffractive surface and generates a first scattered intensity distribution. The optical source further provides a second beam, which illuminates a second region of the diffractive surface and generates a second scattered intensity distribution. A plurality of detectors can be positioned about the diffractive surface to detect the first and second scattered intensity distributions. The detectors are coupled to a detection circuit. The detectors provide the detection circuit with information related to the detected first and second scattered intensity distributions. The detection circuit processes the information related to the detected first and second scattered intensity distributions to determines if a flaw is present on the diffractive surface.
The system further includes a movable mounting table that is adapted to securely retain an object holder. The object holder carries an object, the diffractive surface of which is to be inspected. The mounting table, which has the object holder and object under inspection, can be moved with respect to the first and second beams to generate a scan pattern on the diffractive surface.
The optical source includes a first mirror that receives and redirects an optical beam. The optical beam can be provided by an optical light emitter. The optical beam can be redirected by the first mirror to provide the first beam, which illuminates the first region of the diffractive surface. Similarly, the optical source can further include a second mirror that receives and redirects the optical beam provided by the optical light emitter to provide the second beam. The second beam can illuminate the second region of the diffractive surface. The first and second mirrors can each include an off-axis parabolic mirror. The optical source can also include a pivotable mirror that is oriented to receive the optical beam provided by the optical light emitter.
The pivotable mirror can be pivoted to a first position to redirect the optical beam to the first mirror and the pivotable mirror can be pivoted to a second position to redirect the optical beam to the second mirror.
The optical light emitter can include an ultra violet laser. The ultra-violet laser can project an elliptical beam spot on the diffractive surface. Additionally, the ultraviolet laser beam can be controlled to impinge on the diffractive surface at an angle of approximately 60xc2x0 from normal to the surface. The beam width can be at least as large as the beam trace pitch to ensure inspection of the regions between revolutions of the sample. The beam trace pitch can be no greater than approximately 3 micrometers.
The plurality of detectors can include a first detector which can be positioned at a first location proximate the diffractive surface to detect the intensity level of the scattered intensity distribution at the first location. A second detector can be positioned at a second location proximate the diffractive surface to detect the intensity level of the scattered intensity distribution at the second location. A third detector can be positioned at a third location proximate the diffractive surface to detect the intensity level of the scattered intensity distribution at the third location. In addition, the first, second and third detectors can be positioned about the diffractive surface at locations where the intensity level of the first and second scattered intensity distributions from the surface pattern is expected to be below a threshold intensity level.
The first beam can be controlled to illuminate the first region defined on the diffractive surface which includes a first group of angular sectors ranging from approximately 342.5xc2x0-22.5xc2x0, 67.5xc2x0-112.5xc2x0, 157.5xc2x0-202.5xc2x0 and 247.5xc2x0-292.5xc2x0. The second beam can be controlled to illuminate the second region defined on the diffractive surface which includes a second group of angular sectors ranging from approximately 22.5xc2x0-67.5xc2x0, 112.5xc2x0-157.5xc2x0, 202.5xc2x0-247.5xc2x0 and 292.5xc2x0-342.5xc2x0.
The detection circuit includes an analog signal processing circuit which is coupled to the detectors. The analog signal processing circuit is further coupled to a digital signal processing circuit. The digital signal processing circuit is further coupled to a computer control and data storage unit. The analog signal processing circuit receives information related to the first and second scattered intensity distributions from the detectors and provides the information to the digital signal processing circuit. The digital signal processing circuit determines a minimum detected intensity level associated with the first and second scattered intensity distributions detected by the detectors. The digital signal processing circuit can process the minimum detected intensity level to determine if a flaw is present on the diffractive surface as well as to determine flaw size.
An encoder defined on the mounting table provides information related to the position of the illuminated region on the diffractive surface. This information can be provided to the detection circuit to enable the detection circuit to further determine the relative location of a detected flaw on the diffractive surface. The location and other information related to the flaws detected on the diffractive surface can be further processed and/or stored in the computer control and data storage unit defined on the detection circuit.
The optical inspection system can further include a display that displays the flaws and their locations.
The mounting table can include a rotatably mounted plate holder and a slideable translation stage. The mounting table can rotate and translate the object holder, which carries the object that includes the diffractive surface under inspection, to establish the scan pattern defined on the diffractive surface. The scan pattern can include a spiral trace that has a plurality of revolutions of the first and second beams on the diffractive surface. The plate holder can be coupled to a rotation control circuit that controls rotation of the plate holder. The translation stage can be coupled to a translation control circuit that controls the linear motion of the translation stage.
The method of using the optical inspection system to inspect a diffractive surface containing surface patterns to detect flaws on the diffractive surface can include illuminating a first region of the diffractive surface with a first beam to generate a first scattered intensity distribution; illuminating a second region of the diffractive surface with a second beam to generate a second scattered intensity distribution; detecting an intensity level of the first and second scattered intensity distributions generated by the first and second beams, the intensity level being detected at a plurality of locations about the diffractive surface; establishing a minimum detected intensity level; processing the minimum detected intensity level to determine if a flaw is present; and moving the diffractive surface to generate a scan pattern on the diffractive surface, the scan pattern covering the entire diffractive surface.
Processing the minimum detected intensity level further includes indicating the absence of a flaw on the illuminated region of the diffractive surface when the minimum detected intensity level is below a predetermined threshold level and indicating the presence of a flaw on the illuminated region of the diffractive surface when the minimum detected intensity level exceeds the predetermined threshold intensity level.
Illuminating the first region of the diffractive surface with the first beam includes illuminating a first group of predetermined angular sectors defined on the diffractive surface. Illuminating the second region of the diffractive surface with the second beam includes illuminating a second group of predetermined angular sectors defined on the diffractive surface.
Illuminating the first group of predetermined angular sectors defined on the diffractive surface with the first beam can include projecting an elliptical beam spot onto the diffractive surface. In addition, illuminating the first group of predetermined angular sectors defined on the diffractive surface with the first beam can include directing an ultraviolet laser beam to the diffractive surface at an angle of approximately 60xc2x0 from normal to the diffractive surface.
Similarly, illuminating the second group of predetermined angular sectors defined on the diffractive surface with the second beam can include projecting an elliptical beam spot onto the diffractive surface. In addition, illuminating the second group of predetermined angular sectors defined on the diffractive surface with the second beam can include directing an ultraviolet laser beam to the diffractive surface at an angle of approximately 60xc2x0 from normal to the diffractive surface.
Detecting the intensity level of the first and second scattered intensity distributions generated by the first and second beams includes detecting the intensity level of the first and second scattered intensity distribution at a first location proximate the diffractive surface; detecting the intensity level of the first and second scattered intensity distribution at a second location proximate the diffractive surface; and detecting the intensity level of the first and second scattered intensity distribution at a third location proximate the diffractive surface.
Detecting the intensity level of the first and second scattered intensity distributions further includes detecting the intensity level of the first and second scattered intensity distributions at locations about the diffractive surface where the intensity level of the first and second scattered intensity distributions are expected to be below the threshold intensity level.
Moving the diffractive surface to generate the scan pattern on the diffractive surface can further include rotating and translating the object holder and object, which includes the diffractive surface, to establish a spiral trace with a plurality of revolutions of the first and second beams on the diffractive surface. Rotating and translating the object holder and object having the diffractive surface includes overlapping each said revolution of said spiral trace with adjacent revolutions to insure full inspection of the diffractive surface.
Rotating and translating the object holder and object having the diffractive surface can further include spacing said revolutions no greater than approximately 3 micrometers apart.
Moving the object holder and object having the diffractive surface to generate the scan pattern can further include determining the position of the illuminated region on the diffractive surface. Based on the determined position of the illuminated region on the diffractive surface, the location of flaws on the diffractive surface can be determined. The locations and sizes of the flaws detected can thereafter be stored and/or displayed on a display.
The method of using the optical inspection system can further include determining flaw size.